www.solid-earth.net/7/651/2016/ doi:10.5194/se-7-651-2016
© Author(s) 2016. CC Attribution 3.0 License.
A new X-ray-transparent flow-through reaction cell for a
µ
-CT-based concomitant surveillance of the reaction
progress of hydrothermal mineral–fluid interactions
Wolf-Achim Kahl1, Christian Hansen1,a, and Wolfgang Bach1,2
1Department of Geosciences, University of Bremen, Klagenfurter Straße (GEO), 28359 Bremen, Germany 2MARUM, Center for Marine Environmental Sciences, 28359 Bremen, Germany
anow at: Research Group for Marine Geochemistry (ICBM-MPI Bridging Group), Carl von Ossietzky University of
Oldenburg, Institute for Chemistry and Biology of the Marine Environment (ICBM), 26129 Oldenburg, Germany
Correspondence to: Wolf-Achim Kahl (wakahl@uni-bremen.de)
Received: 2 February 2016 – Published in Solid Earth Discuss.: 3 February 2016 Revised: 11 April 2016 – Accepted: 12 April 2016 – Published: 22 April 2016
Abstract. A new flow-through reaction cell consisting of an X-ray-transparent semicrystalline thermoplastic has been developed for percolation experiments. Core holder, tub-ing and all confintub-ing parts are constructed ustub-ing PEEK (polyetheretherketone) to allow concomitant surveillance of the reaction progress by X-ray microtomography (µ-CT). With this cell setup, corrosive or oversaturated fluids can be forced through rock cores (up to ∅19 mm) or powders at pressures up to 100 bar and temperatures up to 200◦C. The
reaction progress of the experiment can be monitored with-out dismantling the sample from the core holder.
The combination of this flow-through reaction cell setup with a laboratory X-rayµ-CT system facilitates on-demand monitoring of the reaction progress of (long-term) hydrother-mal experiments in the own laboratory, keeping interruption times as short as possible. To demonstrate both the suitability of the cell construction material for X-ray imaging purposes and the experimental performance of the flow-through sys-tem, we report the virtually non-existent bias of the PEEK cell setup with distinctive X-ray observations (e.g., differ-ing states of pore filldiffer-ings: air vs. fluid; detection of delicate fabric elements: filigree zeolite crystals overgrowing weath-ered muscovite), and the monitoring of the gypsum/anhydrite transition as a case study of a 4-D fabric evolution.
1 Introduction
Fluid–rock interactions govern the critical mass transfers in-volved in the exchange between the oceanic crust and seawa-ter and the formation of hydrothermal deposits in a range of settings. They also influence hydraulic and rock mechanical properties and are hence relevant to applied geosciences. Un-derstanding these processes requires consideration of multi-ple factors, including changes in porosity distribution, per-meability and their relationship with critical dissolution and precipitation rates. To date, the knowledge of these interde-pendencies is very limited, and we are hence often unable to make reliable predictions of fluid–rock behaviour in natural systems.
ployed µ-CT-derived pore space geometries and experi-mental data from percolation experiments with rock micro-cores to parametrize coupled transport-reaction models (e.g., Flukiger and Bernhard, 2009; Noiriel et al., 2009).
In a number of studies, reaction progress in hydrother-mal fluid–rock interaction experiments was investigated by means of lab-based or synchrotron µ-CT: pore space evo-lution, changes in rock fabric and mineral surface area, as well as hydraulic scale displacement effects in percolation experiments were quantified by comparison of the initial and the post-reaction states. This procedure has been ap-plied to a variety of geological materials and model rock ana-logues of various dimensions, e.g. micro-cores of sandstone (Sell et al., 2013: core ∅47.6 mm, length 46.0 mm), lime-stone (Gouze and Luquot, 2011: core∅9 mm, 18 mm length;
Smith et al., 2013a: core ∅15 mm, 30 mm length; Vialle et
al., 2014: ∅100 mm, length 350 mm), anhydrite (Smith et
al., 2013b: core length 30 mm), limestone analogue (Noiriel et al., 2012: mixed calcite grains and glass beads,∅6.5 mm,
length 12 mm).
All former studies have in common, however, that the sam-ples were only scanned twice, prior to confinement within the reaction cell and at the end of the experiments after a complete dismantling. Reaction concomitant scans were not possible, either because non-transparent X-ray reactor mate-rials were used or because of impracticabilities in introduc-ing the pressure housintroduc-ing into the beam chamber. In case of 4-Dµ-CT studies that rely on more than the initial and the final time step, removing the sample from the experiment, and its reassembly after scan, has to be mastered with mini-mal perturbation of the core. Burlion et al. (2006) performed leaching experiments on concrete samples in a batch reac-tor setup, removing the sample five times from the experi-ment for synchrotron X-raying. Noiriel et al. (2009) scanned a limestone sample before, after 9 h, and after 15 h (end of experiment); for the scans the sample was removed, but re-mained water-saturated. Noiriel et al. (2005) surveyed lime-stone dissolution by comparing three scans, taken after sub-sequent stages of reactive percolation, with the unreacted sample (cores∅9 mm, length 21 mm; samples were removed from the cell for the scans).
In each of these studies, the core was removed from its pressure housing prior to scanning so that disturbance of the core during dismantling/remounting for repeated scan-ning was an issue. This problem can be avoided, if the sam-ple stays mounted for the scan. Likewise, image registra-tion of subsequent scans can be simplified that way.
Cur-theim sandstone, core ∅5 mm, 12 mm length; Berg et al.,
2013: imaging of pore-scale displacement events in Berea sandstone in real-time, core ∅4 mm, 10 mm length;
Fus-seis et al., 2014, presented an X-ray-transparent cell setup for real-time evolution of pore structure and oil/water imbi-bition experiments for cylindrical samples of up∅3 mm).
Most recently, X-ray-transparent cell setups were also pre-sented for laboratory-based X-ray micro-computed tomogra-phy scanners: Bultheys et al. (2015) performed in situ, time-resolved imaging of dynamic water–rock interaction pro-cesses using a PMMA flow cell (drainage of water-saturated Bentheim sandstone by an oil phase: core∅6 mm, 17 mm length) to visualize the time-dependant development of pref-erential flow channels in limestone. Lebedev (2015) reported flooding tests on∅5 mm cores of Bentheim sandstone
us-ing a newly developed low-cost cell composed of a PEEK (polyetheretherketone) body and metal nuts and ferrules. Ott et al. (2012) investigated the injection of supercritical CO2
in Berea sandstone (cores∅10 mm, length 50 mm) and
in-jection of supercritical H2S in limestone (core∅125.4 mm,
length 90 mm), using a layered pressure vessel jacket com-posed of aluminium and two varieties of Cyanate-Ester. Due to the custom-built setup ofµ-CT-scanner and safety cabi-net, the core holder design from Ott et al. (2012) permits a maximum sample length of 0.5 m.
Here, we present a new flow-through reaction cell con-sisting of an X-ray-transparent semicrystalline thermoplas-tic that has been developed for (long-term) percolation ex-periments to allow concomitant surveillance of the reaction progress by X-ray microtomography without dismantling the sample. The cell size facilitates its use in standard CT scan-ner models, while the core size is large enough to permit in-vestigation of intricate reaction textures (e.g., in the course of serpentinization reactions) or complex rock analogues (e.g., to study metasomatic reactions along strong contrasts in chemical potential across different lithologies). The unique material properties of the weakly attenuating, but mechani-cally strong, cell material allow the experimental examina-tion of rock–fluid interacexamina-tions under low to moderate tem-peratures (up to 200◦C) and fluid pressures up to ca. 100 bar
Figure 1. X-ray-transparent PEEK flow-through reaction cell. (a) Schematic illustration of the percolation cell, the location of the core is indicated. Note: the different parts of the assembled cell are labelled as follows: 1 – steel rings, 2 – end caps, 3 – central part, 4 – steel half-rings, 5 – outlet plug of the core holder and 10–32 finger-tight fitting. (b) Photograph of the assembled cell, with PEEK tubing 1/16′′(1.60 mm) O.D. attached. (c) Photograph of an
ongo-ing percolation experiment: the percolation cell is located inside the oven, recharge- and discharge fluids are connected by PEEK cap-illary tubing. The pump system, recharge- and discharge fluid stor-age, and the board with the flow line are installed on a mobile rack. To facilitate the transfer of the percolation experiment from the oven laboratory to the X-ray microtomography laboratory for scanning, the mobile rack is equipped with an uninterruptible power supply. (d)µ-CT transmission image of a gypsum core mounted inside the percolation cell.
2 Materials and methods
2.1 PEEK percolation cell
The semicrystalline thermoplastic material (polyetherether-ketone: PEEK) is commercially available (KTK Kunst-stofftechnik Vertriebs GmbH, Germering, Germany). Since PEEK is machinable, withstands pressure and has a high mechanical strength and hardness over a broad tempera-ture range, an external metal pressure vessel housing is not necessary in the experimental setup. Instead, 20 mm thick
Figure 2. Scheme of the flow line. All tubing, valves etc. are made of PEEK, the pins of the needle valves are made of titanium.
PEEK walls are sufficient to safely maintain pressures up to 100 bar (10 MPa) and temperatures up to 200◦C. We
use two PEEK modifications for two different temperature ranges: PEEK extruded natural (density,ρ: 1.31 g cm3; yield stress/tensile strength,σ: 110 MPa; heat deflection temper-ature, HDT: 152◦C) up to 120◦C, and PEEK mod extruded
(ρ: 1.46 g cm3;σ: 120 MPa; HDT: 293◦C) up to 200◦C. The
percolation cell (Figs. 1 and S1, Supplement) consists of a dumbbell-shaped central part (housing the core holder) and two end caps, which are assembled by screws connecting a set of metallic rings and half-rings (see parts list in Table 1a). The assembled cell measures 210 mm in height and 100 mm in diameter. The central part of the dumbbell contains the core holder and is radiographed during the scan (Fig. 1d). It is composed of a hollow cylinder of 22 mm width with 20 mm thick walls. Both in- and outlet plugs of the core holder are 19 mm in diameter and 30 mm long; they are connected to the PEEK capillary via PEEK 10–32 finger-tight fittings. Where the capillary passes the PEEK cell end caps, the cell is sealed by another finger-tight fitting.
The reaction cell can accommodate rock cylinders of 19 mm diameter and up to 50 mm length that are mantled with FEP (fluoroethylene propylene; Adtech Polymer Engi-neering ltd., Stroud, UK) heat-shrink sleeves. PEEK capil-lary tubing (Upchurch Scientific, 1/16′′O.D.×0.020′′ I.D.;
1.60 mm O.D.×0.5 mm I.D., see Table S1 in the Supple-ment) provides fluid circulation through the percolation unit. Spiderweb-type groove patterns on the end-faces of the core holder’s inlet and outlet plugs ensure an even distribution of the fluids over the entire face of the core before water en-ters or leaves the plug. An optional external application of additional mantle pressure (overburden) on the heat-shrink sleeve ensures a well-confined maximum fluid flow through the rock core, even at elevated experimental pressure levels.
by KELLER®pressure sensors (series 33X, floating piezore-sistive transducer, range 0–100 bar, material in contact with media: Hastelloy, Viton) and the pressure readings were pro-tocolled by a custom-made Labview® routine. Fluid sam-ples can be retrieved for chemical analyses episodically from the outflow line without disrupting the conditions within the reaction cell: to keep the cell pressure constant during the fluid extraction procedure, the sampling unit is located after a back-pressure valve. This procedure allows the experimen-talist to identify the point in time when condition within the cell are at steady-state and compute saturation states with re-gards to different actual and hypothetical solid reaction prod-ucts.
During the ongoing percolation experiment, the perco-lation cell is located inside the oven, and recharge- and discharge fluids are connected by PEEK capillary tubing (Fig. 1c). The pump system, recharge- and discharge fluid storage, and the board with the flow line are installed on a mobile rack. To facilitate the transfer of the percolation ex-periment from the oven laboratory to the X-ray microtomog-raphy laboratory for scanning, the mobile rack is equipped with an uninterruptible power supply.
2.2 Sample materials
The following samples have been chosen for initial test runs aimed at demonstrating the unbiased X-ray observations of temporal changes in rock samples mounted in shrink sleeves within the percolation cell. To assess differing states of pore fillings (air vs. fluid), a quartz-dominated sandstone (local-ity: Gildehaus, Romberg quarry, Germany; formation: Valan-gin, Lower Cretaceous) has been partially saturated with wa-ter. To make a sandstone sample with both water- and air-filled pore spaces, we first flooded the sample completely with water at 50 bar fluid pressure, and then de-pressurized and disconnected the tubing to permit evaporation towards the state we eventually scanned. To demonstrate the sen-sitivity of in situ cell X-ray observations with respect to subtle differences in mineralogy and delicate textures, we mounted and scanned a mineralogically diverse tertiary sand-stone (“Idaho Gray sandsand-stone”, Idaho, USA; Idaho forma-tion, tertiary). This sandstone was previously characterized by M. Halisch and S. Kaufhold (personal communication, 2015) as coarse grained sandstone with more than 75 % of the grains ranging between 0.35 and 1.1 mm in size. It consists mostly of quartz (∼80 %) besides heavily weathered mus-covite (∼11 %) and K-feldspar (∼4 %). For a grain fraction
vestigated by concomitantµ-CT monitoring of two perco-lation experiments over the course of 77 and 35 days. The experimental conditions were 110◦C, 45 bars fluid pressure,
and a flow rate of 0.05 mL min−1 of a partially
gypsum-saturated fluid (1.5 g L−1dissolved gypsum; saturation state
is 2 g L−1).
2.3 X-ray microtomography surveillance
The X-ray microtomography scans aimed at assessing the applicability of the percolation cell were performed using a CT-ALPHA system (ProCon, Germany) at the Department of Geosciences, University of Bremen, Germany. The sand-stone micro-cores mounted in shrink sleeves inside the per-colation cell (Fig. 1) were scanned with a beam intensity of 100 kV, an energy flux of 300 µA, and using an aluminium filter in 360◦-rotation scans conducted with a step size of
0.225◦. The gypsum (selenite) crystals were mounted in a
similar fashion and scanned with a beam intensity of 130 kV, an energy flux of 350 µA, and a copper filter, using a 360◦
rotation with a step size of 0.3◦. Reconstruction of the
spa-tial information on the linear attenuation coefficient in the samples was done with the Fraunhofer software VOLEX, us-ing a GPU-hosted modified Feldkamp algorithm based on filtered backprojection (Feldkamp et al., 1984). Filtering of the raw data, volume reconstruction, segmentation, and ren-dering were done using Avizo 9.0.1 (FEI).
3 Results
Our results demonstrate that the placement of the micro-cores in the PEEK flow-through cell setup, mounted in shrink sleeves, does not bias the results of X-ray investiga-tion and allows the concomitant surveillance of hydrother-mal mineral–fluid interactions by repeatedµ-CT scans with-out dismantling the sample. This assessment is based on data collected for a set of selected samples presented in following sections, which demonstrate the power of the developed ex-perimental setup for facilitating investigations of states and fabrics of water–rock systems undergoing active reaction.
3.1 Detection of different states of pore filling in the mounted experiment
Figure 3. Reconstructed images of a partially saturated, quartz-rich sandstone sample (Gildehaus, Romberg quarry;∅19 mm) demon-strating the virtually non-existent interference of the PEEK cell setup with distinctive X-ray observations. The samples are scanned inside the assembled cell, surrounded by the PEEK cell material and a shrink sleeve. (a) Unfiltered reconstruction. The distinction be-tween air-filled (black) and fluid-filled (dark grey) pores is already straightforward (voxel size is 8.95 µm). (b) The reconstructed im-age, treated with a 2-D non-local means filter, allows definite seg-mentation of the pore fillings. Note the shrink sleeve in the upper left of each image.
can easily be distinguished from fluid-filled pores (dark gray) in the unfiltered image (Fig. 3a, voxel size is 8.95 µm). Fil-tering of the data set (with non-local means 2-D, Avizo; see Fig. 3b) allows definite segmentation of the different pore fillings and the rock material by means of gray level thresh-olding.
3.2 “Region-of-interest” tomography of parts of a mounted sample which exceeds the detector field of view
The reconstructed images of Idaho Gray sandstone sample (∅19 mm), scanned inside the assembled cell, surrounded
by the PEEK cell material and a shrink sleeve, clearly show the characteristic X-ray attenuation of the individual phases (Fig. 4). Aiming for an optimal resolution, the field of view for this scan covered approximately half the length of the sample and roughly two thirds of the sample diameter. Del-icate fabric elements, such as weathered muscovites and feldspars, can be recognized (voxel size is 6.22 µm) already in the unfiltered reconstructed images (Fig. 4a). Moreover, the recognition of structures, such as filigree zeolite crystals (Fig. 4b and c, both filtered with 3-D Sigma followed by 2-D non-local means) overgrowing weathered muscovite demon-strates the suitability of the cell construction material for high resolution “region-of-interest tomography” (scan and recon-struction of “large” objects exceeding the detector field of view). In this regard, the highest possible magnification will be limited by geometrical issues arising from the percola-tion cell dimensions and the size of the X-rayµ-CT scanner housing.
Figure 4. Reconstructed images of a micro-core of Idaho Gray sandstone (∅19 mm), scanned inside the assembled cell, sur-rounded by the PEEK cell material and a shrink sleeve. (a) Un-filtered reconstruction. Delicate fabric elements such as weathered muscovites and feldspars can be recognized (voxel size is 6.22 µm). (b) The filtered image (3-D Sigma followed by 2-D non-local means, both Avizo 9.0.1) of the same slice. (c) 3-D volume re-construction, with the area defined in (a) being the top layer. The recognition of structures such as filigree zeolite crystals overgrow-ing weathered muscovite impressively demonstrates the suitability of the cell construction material for “region-of-interest tomogra-phy” (scan and reconstruction of “large” objects exceeding the de-tector field of view).
3.3 Concomitant X-ray surveillance of reaction progress without unmounting or dismantling the sample
To assess the experimental performance of the flow-through system in terms of the feasibility of repeated, concomitant scans of long-term percolation experiments, we monitored the gypsum/anhydrite transition as a case study of a 4-D fab-ric evolution. Two percolation experiments were performed (experiment A for 77 days and experiment FTGy-B for 35 days) using artificially fractured selenite crystals that were subjected to a partially gypsum-saturated fluid (1.5 g L−1), percolating at a flow rate of 0.05 mL min−1 at
110◦C and 45 bar (4.5 MPa) fluid pressure. To enhance the
formation of flow paths within the selenite micro-core, pure H2O was initially used as recharging fluid in the
Figure 5. 4-D series of the conversion from gypsum to anhydrite (110◦C, fluid pressure 45 bar (4.5 MPa), direction of flow: upward).
In figures (a)–(c) the cross sections are cut at slightly different depths (voxel size is 23.18 µm). (a) Gypsum single crystal (selen-ite, Morocco), initial state, artificially fractured. (b) Intermediate state, after 7 days percolation of gypsum-undersaturated water (pure H2O) to increase permeability. (c) Late stage of experiment after
further 70 days of percolation of a partially gypsum-saturated fluid (1.5 g L−1). Gypsum has almost entirely been replaced through the
growth of secondary anhydrite needles. (d) 3-D model of segmented pore space, anhydrite and gypsum. Note: (i) white arrowheads in (a)–(c), annotated to the core cross section, mark the position of the single slice shown below; (ii) yellow double arrowheads in (a)– (c) mark the position of the initial artificial fracture within the slice.
state with most of the gypsum starting material converted to anhydrite after further 70 days of percolation of the gypsum psaturated experimental fluid (Fig. 5c). 3-D-analyses re-veal that the conversion of gypsum to anhydrite (see illustra-tion in Fig. 5d) was accompanied by an increase of porosity from 3.6 to 27.7 vol. % from the permeability-enhanced state (ii) (Fig. 5b) to the final state (iii) (Fig. 5c).
By concomitantµ-CT surveillance, a correlation between anhydrite growth and the formation and evolution of flow paths can be documented. In the course of the second per-colation experiment FTGy-B (35 days, see Fig. S2), a series of eight µ-CT scans were performed at several time steps (reaching from 0 to 35 days after the beginning of the exper-iment, as denoted in Fig. S2). The formation and broadening of a main cavity (“worm hole”) is clearly visible from the set of successive volume reconstructions. The individual µ-CT slices for each time step reveal the continued growth of an-hydrite needles within the selenite. The nuclei of the newly formed anhydrite are located in apparent association with fractures in the selenite (see Fig. S2, stage after 10 days).
A comparison of fabric and mineralogy produced in both selenite percolation experiments provides additional infor-mation on the kinetic impact of different cooling rates on the mineralogy of the final run product. While experiment
fast-cooling run FTGy-A. In contrast, the slow cooling of run FTGy-B resulted in an extensive secondary conversion of the newly formed anhydrite needles to gypsum.
4 Conclusions
The X-ray-transparent reaction cell setup presented in this work holds great potential for fostering the knowledge about the interdependencies between changes in porosity distri-bution, permeability and their feedback relationship in the course of dissolution and precipitation reactions.µ-CT-based surveillance of percolation experiments can contribute in multiple ways to achieving a more sophisticated understand-ing of fluid–rock interactions:
1. the consideration of true 3-D spatially resolved µ-CT data of real pore space geometries will facilitate the for-mulation of relevant pore space models for transport re-actions;
2. the identification of preferred growth- or dissolution sites can be correlated to preferential fluid pathways; 3. analysis of changing fluid compositions, changing
min-eral chemistry and volume proportions, can be used for the parametrization of coupled reaction–transport mod-els;
4. the 4-D evolution of rock fabric and mineralogy, in par-ticular the dissolution or precipitation rate spectra de-rived from the experiment byµ-CT-based monitoring, can be treated as benchmarks for computer models and can be used for parameterizing such coupled models with increased predictive power.
Information about the Supplement
The following additional figures and table have been sub-mitted as supplementary material with this manuscript: Fig-ure S1 – Collage of photographs featuring the components of the cell. The labels correspond to the positions in Table S1; Figure S2 – Documentation of formation and evolution of flow paths in the course of gypsum dissolution and anhydrite growth by concomitantµ-CT surveillance (series of eight µ-CT scans); Table S1 – Cell assembly and flow line compo-nents, parts list.
The Supplement related to this article is available online at doi:10.5194/se-7-651-2016-supplement.
Acknowledgements. We are grateful to the people that have
con-tributed to this development: first of all to Georg Nover for his quali-fied and cordial discussions concerning cell construction details. We thank Norbert Schleifer & Volker Fendrich and Robert Hinkes & Volker Feeser for helpful insights into their laboratory materials and methods. We are greatly indebted to Elke Sorgenicht, forewoman of the mechanical workshop, and her staff for their brilliant techni-cal realization. We will remember Matthias Lange and highly value his input on flow path issues and Labview programming. Thanks also go to Martin Kölling and Katja Beier for discussions in the early phase of this project. Matthias Halisch and Stephan Kaufhold are acknowledged for providing both samples and characterizations of Idaho sandstone, Norbert Schleifer is thanked for providing an aliquot of Gildehaus sandstone. We gratefully acknowledge the re-views of G. Nover and H. Steeb.
This study was funded through a grant of the DFG to WB within a Reinhart-Koselleck Project BA 1605/10-1.
The article processing charges for this open-access publication were covered by the University of Bremen.
Edited by: M. Halisch
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